Obviously the community for a launch console is going to be smaller, there are 80+ million Xbox 360's in the world, and usually 12-15 million buy COD for 360 every year. Xbox One will probably sell between 2-4 million this year, and I'd expect a good amount of Xbox One owners to buy COD Ghosts, so you'll see 100-200k online, maybe more.
Xbox One is the only console to offer dedicated servers though, so I think that will draw a lot of people to the console.